unix: add //go:build lines when generating files

The //+go:build lines were added by running Go 1.17 gofmt in CL 294490.
Make sure to retain them when re-generating files in this package.

For golang/go#41184.

Change-Id: Ia418b8c0eb4291a0db3f4bb97685bde78d0ff016
Reviewed-on: https://go-review.googlesource.com/c/sys/+/296889
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/linux/mkall.go

@@ -598,6 +598,7 @@ func generatePtracePair(arch1, arch2, archName string) error {
 	buf := bufio.NewWriter(f)
 	fmt.Fprintf(buf, "// Code generated by linux/mkall.go generatePtracePair(%q, %q). DO NOT EDIT.\n", arch1, arch2)
 	fmt.Fprintf(buf, "\n")
+	fmt.Fprintf(buf, "//go:build linux && (%s || %s)\n", arch1, arch2)
 	fmt.Fprintf(buf, "// +build linux\n")
 	fmt.Fprintf(buf, "// +build %s %s\n", arch1, arch2)
 	fmt.Fprintf(buf, "\n")

unix/linux/mksysnum.go

@@ -25,8 +25,13 @@ func cmdLine() string {
 	return "go run linux/mksysnum.go " + strings.Join(os.Args[1:], " ")
 }
 
-// buildTags returns build tags
-func buildTags() string {
+// goBuildTags returns build tags in the go:build format.
+func goBuildTags() string {
+	return fmt.Sprintf("%s && %s", goarch, goos)
+}
+
+// plusBuildTags returns build tags in the +build format.
+func plusBuildTags() string {
 	return fmt.Sprintf("%s,%s", goarch, goos)
 }
 
@@ -129,12 +134,13 @@ func main() {
 	}
 	err = s.Err()
 	checkErr(err)
-	fmt.Printf(template, cmdLine(), buildTags(), text)
+	fmt.Printf(template, cmdLine(), goBuildTags(), plusBuildTags(), text)
 }
 
 const template = `// %s
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
+//go:build %s
 // +build %s
 
 package unix